The present invention is directed to the field of ultrafiltration technology, specifically to spiral filtration modules and methods for making the same.
The term “ultrafiltration” as used in the present application is intended to encompass microfiltration, nanofiltration, ultrafiltration, reverse osmosis and gas separation, unless otherwise indicated.
A typical ultrafiltration device comprises a plurality of spiral filtration modules through which a fluid to be filtered passes. Such spiral filtration modules consist of membrane sheets, permeate carriers and feed spacers wound around a permeate carrier tube. The membrane sheets generally comprise a membrane material integrally joined to a backing material. Each membrane sheet is typically folded in half along its width to present two membrane leaves, integrally joined along the fold line in a leaf packet. Membrane leaves in each leaf packet are oriented such that the membrane material sides of the sheet face each other.
Each permeate carrier is sandwiched between two membrane leaves, with one leaf provided by each of two adjacent leaf packets. The permeate carriers and membrane leaves are oriented such that the membrane material sides of the leaves faces away from the permeate carriers. The leaf side edges and the axial edges of the leaves distant from the permeate carrier tube are sealed around the permeate carrier to provide a permeate carrier envelope. The construction of the envelopes allows access to the permeate carriers only from a radial direction through the membrane leaves. A wet adhesive, typically a one-part or two-part epoxy or urethane, is commonly used to achieve the sealing. The permeate carrier material is usually a porous felt or fabric material, as is well-known in the art.
In most spiral filtration modules, each permeate carrier envelope is separated from adjacent permeate carrier envelopes by a feed spacer. The feed spacers are of a relatively large mesh size to accommodate fluid flow. The fluid passes along the feed spacers in a direction parallel to the axis of the permeate carrier tube. The permeate passes through the membrane surface of the permeate carrier envelopes and is directed to holes in the permeate carrier tube by the permeate carriers. Because the permeate carrier envelopes are sealed along the side edges and distant axial edges, fluid flowing through feed spacers sheets cannot access the permeate carrier tube except through the membrane leaves of the permeate carrier envelopes.
Some type of external restraining means such as a hard shell, straps or a bypass screen, or a combination thereof, may be used to hold the spirally wound components in tight formation around the tube. The spiral filtration module is loaded into a housing or pressure vessel which is operated at a slight pressure drop across the module as the fluid being filtered flows through. Concentrate is removed from one end of the module and permeate is removed from the permeate carrier tube.
Many applications of ultrafiltration technology involve food processing where sanitary conditions must be maintained at all times. This necessitates periodic cleaning with relatively harsh chemicals such as, by way of example only, chorine-containing compounds, other oxidizing agents, acids, alkalies and surfactants. The chemicals tend to degrade the membrane material, particularly in the areas that are subject to stress, such as the area along the fold line between the membrane leaves. This area where the two membrane leaves meet is typically referred to as the fold area. This fold area creates mechanical stresses in the membrane sheet and leads to cracking of the membrane sheet and leakage.
It is typical to employ some type of reinforcement in the fold area of the membrane sheet so as to reduce the mechanical stress and prolong the life of the membrane sheet. One technique is the utilization of reinforcing tapes, which are applied at the fold and extend outwardly from the fold a short distance to cover the fold area of the membrane sheet. A second method of membrane sheet reinforcement is to apply an adhesive to the fold area. Commonly used adhesives for such purposes are two-part room-temperature curing polyurethane or epoxy. Another method of membrane reinforcement involves densifying the membrane sheet in the fold area using a fusion process. Although tape, adhesive and fusion reinforcements provide some degree of reinforcement, tape tends to lose its adhesion and the use of adhesives and fusion processes require long cycles time during the manufacturing process and prevent the use of automated winding processes during manufacture.